This invention relates to apparatus and method for determining the rate of flow of a fluid by measuring the electrical potential difference developed in the fluid as the fluid moves through a magnetic field.
In prior art in-line magnetic flow meters, the electrical potential difference developed in the fluid is generally sensed by a pair of electrodes contacting the liquid and spaced apart from each other by the diameter of a round flow sensing passage. A magnetic field generated orthogonal to both the axis between the electrodes and the direction of flow through the sensing passage is provided by two coils of wire located on opposite sides of and outside of the passage. Sophisticated electronics are used to energize the magnetic field, amplify the tiny flow-related signals generated, and reject various noise and drift signal components which would otherwise degrade measurement precision. These meters are characterized by an unobstructed flow passage offering very low pressure drop and high tolerance to solids in the fluid, high measurement precision, high power consumption, and high cost.
In a water metering application for irrigation, where only moderate flow rates are experienced, an unobstructed flow passage is relatively unimportant, but low cost and low power consumption for stand alone battery operation are very important. It is therefore an object of the invention to provide the basis for magnetic flow sensors which, at the expense of flow passage restriction, offer advantages of improved measurement precision, reduced operating power and lower costs.
Various of the above and other objects are attained by magnetic flow sensors made or operated in accordance with various preferred embodiments of the present invention. In one preferred embodiment a magnetic flux generated by two electromagnets having magnetic cores is redirected by magnetic pole pieces so as to be orthogonal to both the axis between the electrodes and to a fluid flow direction. As is known in the magnetic flow metering art, the flux will generate, in the moving fluid, voltage differences proportional to the flow rate of the fluid, the magnitude of the flux and the length of the conductive path between the electrodes. These voltage differences are sensed by at least one, and preferably two pairs of electrodes arranged so that one pair is associated with each location of the pole pieces. In this embodiment, one of the electromagnets is located in a streamlined housing centered within the flow passage so as to confine the flow to a quasi-annular ring, and the other electromagnet is on the outside of the passage. The pole pieces from the two magnets are located a selected distance apart and are aligned to reinforce their radial flux through the annular flow passage at two locations along the flow axis, thereby forming a complete magnetic circuit. At each location of these paired poles, a pair of electrodes is located to sense the corresponding flow generated voltages. An electrically insulating barrier may be used both to provide mechanical support for the housing and electrodes, and to electrically isolate the paired electrodes so that the quasi-annular flow passage provides substantially the only electrical path between those electrodes. That is, flow signals are generated along a circumferential path lying between the streamlined body and the tube. In preferred embodiments, one pair of electrodes is located far enough from the other pair so that their signals have low mutual interaction. This provides a combination of an increased fluid flow velocity, a longer path between electrodes and a highly efficient magnetic circuit. These features enable a magnetic flow sensor to be produced having substantially greater flow-generated signals than is found in the prior art.
The presence of a streamlined housing within a flow passage reduces the cross sectional area of the flow passage and thereby increases the fluid flow rate at the expense of an increased pressure drop. At a fixed magnetic flux in the passage, the increased flow rate produces correspondingly higher electrode voltages than would be measured if the body were not there. Moreover, use of magnetic cores with pole pieces to provide a complete, shielded magnetic circuit concentrates the magnetic flux in the desired area. This arrangement enables a higher magnetic circuit efficiency to be achieved than is the case with commonly used air core magnets. Additionally, the magnetic field is generally confined to the annular flow passage in order to reduce problems of magnetic and electromagnetic compatibility. The use of magnetic cores and pole pieces with prior art magnetic flow meters is generally not practical in larger pipe sizes because the orientation of the field would require a relatively large mass of core material that would increase the size and weight of the meters considerably.
In a preferred flow sensing embodiment, each electrode pair may be used with its own signal amplifying and processing circuitry to provide a flow rate signal. Alternately, signals from multiple pairs may be combined in various ways to provide redundancy and improved measurement precision. Each electrode pair may also be stabilized by short-circuiting the two electrodes of the pair together, or otherwise connecting both of the two electrodes to a common potential during the period when the magnetic field is not present, thereby further helping to reduce measurement errors,
Although it is believed that the foregoing recital of features and advantages may be of use to one who is skilled in the art and wishes to learn how to practice the invention, it will be recognized that the foregoing is not intended to list all of the features and advantages. Moreover, it may be noted that various embodiments of the invention may provide various combinations of the hereinbefore recited features and advantages of the invention, and that less than all of the recited features and advantages of the invention may be provided by some embodiments.